TY - JOUR

T1 - Photoreactions occurring on metal-oxide surfaces are not all photocatalytic. Description of criteria and conditions for processes to be photocatalytic

AU - Emeline, A. V.

AU - Ryabchuk, V. K.

AU - Serpone, N.

PY - 2007/4/15

Y1 - 2007/4/15

N2 - Studies involving the TiO2-assisted photodegradation of organic substances report that the processes are photocatalytic in nature. Yet, no evidence exists confirming such assertions. Previously, we examined the usage of relative photonic efficiencies [N. Serpone, G. Sauve, R. Koch, H. Tahiri, P. Pichat, P. Piccinini, E. Pelizzetti, H. Hidaka, J. Photochem. Photobiol. A: Chem. 94 (1996) 191; N. Serpone, J. Photochem. Photobiol. A: Chem. 104 (1997) 1] and quantum yields Φ [N. Serpone, R. Terzian, D. Lawless, P. Kennepohl, G. Sauve, J. Photochem. Photobiol. A: Chem. 73 (1993) 11] to systematize discrepant claims about process efficiencies. An experimental protocol is now available [N. Serpone, A. Salinaro, Pure Appl. Chem. 71 (1999) 303] to measure true Φ in heterogeneous media. Photoinduced reduction of O2 and photooxidation of H2 occurring on oxidized and reduced surfaces of ZrO2 were recently examined [A.V. Emeline, G.N. Kuzmin, L.L. Basov, N. Serpone, J. Photochem. Photobiol. A: Chem. 174 (2005) 214] to probe the spectral variations of the photoactivity and photo-selectivity of ZrO2 by determining Φ for the two redox reactions at various wavelengths of photoexcitation (200 < λ < 400 nm). Irradiation of ZrO2 in the intrinsic absorption region (λ < 260 nm) led predominantly to photoreduction of O2, whereas photooxidation of H2 predominated on irradiation in the extrinsic spectral region (260 < λ < 360 nm). A difficult task in heterogeneous catalysis and photocatalysis is determination of the actual number of active sites, an issue that has heretofore been elusive to assess the (photo)catalytic activity of a given material in heterogeneous solid-liquid and solid-gas (photo)catalysis. A kinetic description of the three turnover quantities, viz., turnover number (TON), turnover rate (TOR) and turnover frequency (TOF) has been described [N. Serpone, A. Salinaro, A.V. Emeline, V.K. Ryabchuk, J. Photochem. Photobiol. A: Chem. 130 (2000) 83], concluding that turnover quantities are conceptually distinct, with TON and TOR requiring knowledge of the number of active sites on the (photo)catalyst's surface. Apparently, turnovers depend on the nature of the active state of the catalyst and how it is described. The number of surface-active sites on the ZrO2 particle surface have been determined quantitatively (∼1016 active centers) through thermoprogrammed desorption spectroscopy, affording an estimate of TONs for the photooxidation of H2 (TON > 14.5) and photoreduction of O2 (TON > 6.6) on ZrO2 and demonstrating for the first time that a photoreaction occurring on the surface of a metal oxide is truly photocatalytic [A.V. Emeline, A.V. Panasuk, N. Sheremetyeva, N. Serpone, J. Phys. Chem. B 109 (2005) 2785]. Photocoloration of a metal oxide such as ZrO2 (process of photoinduced formation of Zr3+, F- and V-type color centers) during a surface photochemical reaction was also used to assess whether a reaction is photocatalytic. Our study on the influence of simple photoreactions involving the photoreduction of O2, photooxidation of H2, photooxidation of H2 by adsorbed O2, and photoinduced transformation of NH3 and CO2 on the photocoloration of ZrO2 concluded that photoreactions involving NH3 and CO2 are non-photocatalytic processes, in contrast to the photooxidation of H2 which is photocatalytic [A.V. Emeline, G.V. Kataeva, A.V. Panasuk, V.K. Ryabchuk, N.V. Sheremetyeva, N. Serpone, J. Phys. Chem. B 109 (2005) 5175]. In this article, we describe the criteria and conditions by which a photoreaction taking place on the surface of a solid can be said to be photocatalytic by considering both a chemical approach and a physical approach.

AB - Studies involving the TiO2-assisted photodegradation of organic substances report that the processes are photocatalytic in nature. Yet, no evidence exists confirming such assertions. Previously, we examined the usage of relative photonic efficiencies [N. Serpone, G. Sauve, R. Koch, H. Tahiri, P. Pichat, P. Piccinini, E. Pelizzetti, H. Hidaka, J. Photochem. Photobiol. A: Chem. 94 (1996) 191; N. Serpone, J. Photochem. Photobiol. A: Chem. 104 (1997) 1] and quantum yields Φ [N. Serpone, R. Terzian, D. Lawless, P. Kennepohl, G. Sauve, J. Photochem. Photobiol. A: Chem. 73 (1993) 11] to systematize discrepant claims about process efficiencies. An experimental protocol is now available [N. Serpone, A. Salinaro, Pure Appl. Chem. 71 (1999) 303] to measure true Φ in heterogeneous media. Photoinduced reduction of O2 and photooxidation of H2 occurring on oxidized and reduced surfaces of ZrO2 were recently examined [A.V. Emeline, G.N. Kuzmin, L.L. Basov, N. Serpone, J. Photochem. Photobiol. A: Chem. 174 (2005) 214] to probe the spectral variations of the photoactivity and photo-selectivity of ZrO2 by determining Φ for the two redox reactions at various wavelengths of photoexcitation (200 < λ < 400 nm). Irradiation of ZrO2 in the intrinsic absorption region (λ < 260 nm) led predominantly to photoreduction of O2, whereas photooxidation of H2 predominated on irradiation in the extrinsic spectral region (260 < λ < 360 nm). A difficult task in heterogeneous catalysis and photocatalysis is determination of the actual number of active sites, an issue that has heretofore been elusive to assess the (photo)catalytic activity of a given material in heterogeneous solid-liquid and solid-gas (photo)catalysis. A kinetic description of the three turnover quantities, viz., turnover number (TON), turnover rate (TOR) and turnover frequency (TOF) has been described [N. Serpone, A. Salinaro, A.V. Emeline, V.K. Ryabchuk, J. Photochem. Photobiol. A: Chem. 130 (2000) 83], concluding that turnover quantities are conceptually distinct, with TON and TOR requiring knowledge of the number of active sites on the (photo)catalyst's surface. Apparently, turnovers depend on the nature of the active state of the catalyst and how it is described. The number of surface-active sites on the ZrO2 particle surface have been determined quantitatively (∼1016 active centers) through thermoprogrammed desorption spectroscopy, affording an estimate of TONs for the photooxidation of H2 (TON > 14.5) and photoreduction of O2 (TON > 6.6) on ZrO2 and demonstrating for the first time that a photoreaction occurring on the surface of a metal oxide is truly photocatalytic [A.V. Emeline, A.V. Panasuk, N. Sheremetyeva, N. Serpone, J. Phys. Chem. B 109 (2005) 2785]. Photocoloration of a metal oxide such as ZrO2 (process of photoinduced formation of Zr3+, F- and V-type color centers) during a surface photochemical reaction was also used to assess whether a reaction is photocatalytic. Our study on the influence of simple photoreactions involving the photoreduction of O2, photooxidation of H2, photooxidation of H2 by adsorbed O2, and photoinduced transformation of NH3 and CO2 on the photocoloration of ZrO2 concluded that photoreactions involving NH3 and CO2 are non-photocatalytic processes, in contrast to the photooxidation of H2 which is photocatalytic [A.V. Emeline, G.V. Kataeva, A.V. Panasuk, V.K. Ryabchuk, N.V. Sheremetyeva, N. Serpone, J. Phys. Chem. B 109 (2005) 5175]. In this article, we describe the criteria and conditions by which a photoreaction taking place on the surface of a solid can be said to be photocatalytic by considering both a chemical approach and a physical approach.

KW - Criteria and conditions for photocatalysis

KW - Photocatalytic processes

KW - Photocoloration of metal oxides

KW - Photoreactions on metal-oxide surfaces

KW - Titanium dioxide

KW - Zirconium dioxide

UR - http://www.scopus.com/inward/record.url?scp=34047142236&partnerID=8YFLogxK

U2 - 10.1016/j.cattod.2007.02.007

DO - 10.1016/j.cattod.2007.02.007

M3 - Article

VL - 122

SP - 91

EP - 100

JO - Catalysis Today

JF - Catalysis Today

SN - 0920-5861

IS - 1-2

ER -